In magneto-optic disk drives, the recording and erasing of information are conducted by heating magnetic thin-film on the recording surface of a magneto-optic disk to a temperature greater than Curie point through laser light converged by an objective lens, then directing magnetization direction of an area where the laser light is projected in a desired direction by applying an external bias magnetic field. Therefore, the formation of the external magnetic field is an essential requirement.
FIG. 1A shows a typical conventional magneto-optic disk drive 100 which includes a carriage provided with an optical head composed of an optical system 101 and a bias applying means 102. In this magneto-optic disk drive, when the recording and erasing of information are conducted onto the single recording surface of a magneto-optic disk D1, a desired number of blocks are erased in the first rotation of the magneto-optic disk drive D1 and then data are recorded in the next rotation. Thus, it cannot efficiently process a number of data since the erasing and recording process needs the two rotations of the magneto-optic disk D1.
To improve efficiency in the erasing and recording process, another magneto-optic disk drive has been developed. The magneto-optic disk drive 110, as shown in FIG. 1B, comprises an erasing optical head composed of an optical system 111 and a bias applying means 113 and a recording and reproducing optical head composed of an optical system 112 and a bias applying means 114, where their carriages are disposed symmetrically from the center of a magneto-optic disk D1. In the magneto-optic disk drive 110, the recording and erasing of information can be conducted in one rotation of the magneto-optic disk D1.
However, to a magneto-optic disk with the upper and lower recording surfaces, the magneto-optic disk drive 110 can only access one of the two recording surfaces of the magneto-optic disk. To access the other recording surface, an operator has to change the side of the magneto-optic disk or an external changer or the like is necessary. To solve this problem, a magneto-optic disk drive 120, where two carriages are, as shown in FIG. 2, disposed symmetrically around the rotation center of the magneto-optic disk D and the upper and lower recording surfaces are accessed individually through an erasing optical head and a recording and reproducing optical head provided on the carriages, has been developed.
In this type of magneto-optic disk drive, considering the possibility that biasing magnetic fields of the optical heads may be canceled with each other when the two carriages are disposed close to each other, the two carriages need to be disposed symmetrically around the rotation center of the magneto-optic disk D.
FIGS. 9A and 9B show a typical form of a cartridge 5 including a magneto-optic disk 1 with two recording surfaces. The cartridge 5 is formed as a planar box and is provided with windows located at common positions of the upper and lower surfaces to expose the upper and lower recording surfaces of the magneto-optic disk 1 by moving a slidable shutter 4. Thus, to access the magneto-optic disk 1 received in the cartridge 5, the two carriages of a magneto-optic disk drive have to be disposed at positions opposite to each other. Therefore, the magneto-optic disk drive 120 in FIG. 2 cannot be used to access simultaneously both the recording surfaces of the cartridge-type magneto-optic disk.
Furthermore, the entire magneto-optic disk drive must be enlarged since the carriages are disposed apart from each other.